1. Field of the Invention
The present invention relates to an optical data reader. More particularly, the invention relates to a device used for reading a laser beam reflected from data such as bar codes.
2. Related Art
Recently, bar code readers have been used in various fields, such as product control and sales control. For example, a laser scanning-type device that moves a beam across a bar code label using a polygonal mirror or galvanized mirror. The reflected beam is converted into an electrical signal using photodiodes and the bar code label is read.
The scanner used must have sufficient resolution for scanning the beam across the bar code and maintaining a minimum width of the beam such that the bar code is irradiated by the beam. The width of the bar code must be larger than the beam's diameter. Thus, such a laser is restricted in the number of bar codes it can read. The range of applicable bar codes centers on the beam waist position where the beam width is the smallest.
In recent years, however, there has been an increased need for bar code readers having a wide reading range such as contact readers, close range readers, and far range readers.
Various alternatives have been proposed to alleviate the problems noted above. For example, in Japanese Unexamined Patent Publication No. 4-195271, an automatic focus-type laser scanner measures the distance from the bar code label to the scanner with a length gauge. This device changes the focal length of the laser in accordance with the measured length. Japanese Unexamined Patent Publication No. 2-7182 discloses a method of adjusting the focal point by movement of a lens mechanism using a motor so that amplitude of the bar code signal is the maximum based on the bar code signal from the photoelectric conversion section.
Japanese Unexamined Patent Publication No. 4-195271 requires the use of a device for controlling the length gauge and the focal length according to the data from the length gauge. Also, the possibility existed that such targets as printed bar codes on cylindrical cans could not be read with great accuracy. In Japanese Unexamined Patent Publication 2-7182, no length gauge was used. Rather, a driving means, such as a motor, mechanically moves the lens, thus making the required mechanism quite complex.